Quinacridone (QA) is known to exist in three crystal phases. The alpha phase, described in U.S. Pat. No. 2,844,484 and gamma phase described in U.S. Pat. Nos. 2,844,581 and 2,969,366 forms are a bluish red color. The beta form described in U.S. Pat. Nos. 2,844,485 and 4,857,646 is violet. The alpha quinacridone crystal form is not commercially valuable because it is not heat stable. See W. Herbst and K. Hunger, “Industrial Organic Pigments”, VCH Publishers, Inc., 1997, page 464.
It is well known in the art that organic pigments, such as quinacridones, as synthesized, are generally unsuitable for use as pigments and must be further processed to develop the requisite pigmentary properties such as particle size, particle shape, polymorphic phase, and tinctorial strength.
In order to obtain the color properties required for a particular application, the pigment crude must be converted to a pigmentary grade with a proper tint strength, transparency or opacity for a particular application. The effectiveness of a given pigment type in imparting color is dependent upon it's particle size in dispersion. Thus, color strength, transparency and opacity are all properties that are highly dependant on particle size. Consequently, crude organic pigments undergo one or more finishing or conditioning steps that require particle size reduction. See, for example R. B. McKay, “Control of the Application Performance of Classical Organic Pigments” in JOCCA, 89–93.
Thus, the crude beta- or gamma-quinacridone usually undergoes a particle size reduction process. During this particle reduction of the beta or gamma quinacridone, the beta or gamma will tend to convert to the alpha crystal form depending on the milling conditions without a crystal phase inhibitor. As mixing of the alpha-quinacridone with either the beta or gamma phase changes product color shade and decreases heat stability of the final finished pigment, inhibition of this conversion during milling is to be avoided.
EP 517662 and U.S. Pat. No. 5,281,269 describe an aqueous milling process of modifying beta-quinacridone (QA) with base and phase-transfer catalyst.
EP 1020497 describes the color property of the mixed crystal phase pigment with 2,9-dichloroquinacridone.
EP 799863 describes the preparation of beta-phase quinacridone by conversion of alpha-phase quinacridone.
EP 517663 and EP 517662 describe a process of preparing magenta colour beta-1 form quinacridone pigment by either dry milling of beta-quinacridone crude, or milling of beta-quinacridone crude in the presence of water and alcohol.
EP 305328 describes a new magenta color beta-quinacridone that has average particle size over 0.1 microns.
However, none of the above references disclose the use of 2,9-dichloroquinacridone as an alpha-quinacridone crystal phase inhibitor in beta quinacridone particle size reduction process.
Surprisingly, it has been discovered that the beta-quinacridone crystal phase can be preserved during particle size reduction by the addition of 2,9-dichloroquinacridone during the finishing process. The beta-quinacridone product obtained from this process has blue shade violet color that is not achievable when alpha-quinacridone exists in the product.
Furthermore, it has also been discovered that the same crystal phase inhibitor, 2,9-dichloroquinacridone can also be used with the gamma-quinacridone during the particle size reduction to prevent the formation of alpha-quinacridone. Gamma-quinacridone has red color that can be shifted to yellow or bluer shade. Particle size reduction shifts the color to a bluer shade with/without alpha-quinacridone. Beta-quinacridone develops violet color as particle size is reduced. Without alpha-quinacridone, the beta shifts to a bluer violet shade. Thus, a saturated violet color product for beta and red color for gamma is produced with better pigment properties for coatings, plastics and ink applications is achieved for both beta-quinacridone and gamma-quinacridone by milling in the presence of 2,9-dichoroquinacridone as a crystal phase inhibitor.